1. Field of the Invention
The present invention relates to techniques for adjusting the loss or attenuation caused by the curvature of an optical fiber and, more particularly, to a method and an apparatus for adjusting an intensity loss of an optical signal transmitted through the optical fiber, as well as to an optical transmission system using the same.
2. Description of the Related Art
In a multiple-relay wavelength division multiplexing transmission system, a large number of optical multiplexers/demultiplexers, optical amplifiers and the like are used. These optical devices are constituted in such a manner that two or more optical components are connected by fiber fusion splicing. To prevent variations in loss among multiple principal signals, adopted is loss increase splicing, which intentionally gives a loss.
To accomplish this intentional increase of a loss, generally employed is a method by which the cores of spliced fibers are intentionally misaligned when fusion splicing is performed. However, although this loss increase splicing ensures the quality of transmission on one hand, an expensive optical fiber fusion splicer is needed, as well as many hours of work required, to splice fibers with their cores misaligned, on the other hand. Therefore, there are problems such as increased manufacturing costs and lead times.
For another method, Japanese Patent Application Unexamined Publication No. 2001-228337 discloses an optical attenuator which utilizes the phenomenon that the amount of loss in an optical fiber depends on the curvature radius of the optical fiber. This optical attenuator includes: a main body composed of a base with a plurality of projections thereon; and an optical fiber placed on a face of the base, bent in a predetermined circular shape by being brought in contact with the projections. According to an optical-fiber loss adjusting method employed in this optical attenuator, a coarse adjustment is first made in which the amount of optical attenuation per turn of the optical fiber is large with a reduced radius of curvature of the optical fiber, and thereafter, a fine adjustment is made in which the amount of optical attenuation per turn of the optical fiber is reduced by increasing the curvature radius of the optical fiber. In this manner, the level of an optical signal, or the amount of optical attenuation, is adjusted by setting two curvature radii or more, without changing the number of turns of the optical fiber.
However, the optical attenuator employing this conventional method has such a structure that the optical fiber is placed on the face of the seat while being wound round to make double circles, one with a lager radius of curvature and the other with a smaller radius of curvature, the shapes of which are maintained by a support member. Accordingly, there are possibilities of the occurrence of an unexpected loss (microbend) and the loosening of the optical fiber, depending on the shape of the support member where the circle with the larger radius of curvature is connected to the circle with the smaller radius of curvature.